Light-emitting element, light-emitting device, and electronic device

ABSTRACT

It is an object of the present invention to provide a light-emitting element with high light emission efficiency. It is another object of the present invention to provide a light-emitting element with a long lifetime. A light-emitting device is provided, which includes a light-emitting layer, a first layer, and a second layer between first electrode and a second electrode, wherein the first layer is provided between the light-emitting layer and the first electrode, the second layer is provided between the light-emitting layer and the second electrode, the first layer is a layer for controlling the hole transport, the second layer is a layer for controlling the electron transport, and a light emission from the light-emitting layer is obtained when voltage is applied to the first electrode and the second electrode so that potential of the first electrode is higher than potential of the second electrode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a current excitation typelight-emitting element. In addition, the present invention relates to alight-emitting device and an electronic device each having thelight-emitting element.

2. Description of the Related Art

In recent years, research and development have been extensivelyconducted on light-emitting elements using electroluminescence. In abasic structure of such a light-emitting element, a substance having alight-emitting property is interposed between a pair of electrodes. Byapplying voltage to this element, light emission can be obtained fromthe substance having a light-emitting property.

Since such a light-emitting element is a self-light-emitting type, ithas advantages over a liquid crystal display such as higher visibilityof a pixel and unnecessity of a backlight. Accordingly, such alight-emitting element is considered suitable as a flat panel displayelement. In addition, other advantages of such a light-emitting elementare that it can be manufactured to be thin and lightweight and theresponse speed is very high.

Furthermore, since such a light-emitting element can be formed into afilm shape, surface light emission can be easily obtained by forming alarge-area element. This is a feature that is difficult to be obtainedfrom a point light source typified by a filament lamp and an LED or alinear light source typified by a fluorescent light. Therefore, thelight-emitting element has a high utility value as a plane light sourcethat can be applied to lighting or the like.

Light-emitting elements using electroluminescence are classified broadlyaccording to whether they use an organic compound or an inorganiccompound as a substance having a light-emitting property.

When an organic compound is used as a substance having a light-emittingproperty, electrons and holes are injected into a layer including anorganic compound having a light-emitting property from a pair ofelectrodes by voltage application to a light-emitting element, so thatcurrent flows therethrough. Then, these carriers (electrons and holes)are recombined; thus, the organic compound having a light-emittingproperty is brought into an excited state. The organic compound having alight-emitting property returns to a ground state from the excitedstate, thereby emitting light. Based on this mechanism, such alight-emitting element is referred to as a current-excitationlight-emitting element.

Note that the excited state of an organic compound can be a singletexcited state or a triplet excited state, and light emission from thesinglet excited state is referred to as fluorescence and light emissionfrom the triplet excited state is referred to as phosphorescence.

As for such a light-emitting element, there are many problems dependingon materials in improving element characteristics, and improvement inelement structure, development of materials, and the like have beenconducted to overcome the problems.

For example, in Non-Patent Document 1 (Tetsuo TSUTSUI, and eight others,Japanese Journal of Applied Physics vol. 38, L1502 to L1504, (1999)), ahole-blocking layer is provided so that a light-emitting element using aphosphorescent material efficiently emits light. However, as describedin Non-Patent Document 1, a hole-blocking layer has poor durability, andthe light-emitting element has a very short lifetime. Thus, developmentof a light-emitting element with high light emission efficiency and along lifetime has been desired.

SUMMARY OF THE INVENTION

In view of the foregoing problems, it is an object of the presentinvention to provide a light-emitting element with high light emissionefficiency; a light-emitting element with a long lifetime; alight-emitting device and an electronic device with high light emissionefficiency; and a light-emitting device and an electronic device with along lifetime.

As a result of diligent studies, the present inventors have found that alight-emitting element with high light emission efficiency can beobtained by providing a layer for controlling the carrier transport. Inaddition, the present inventors have also found that a light-emittingelement with a long lifetime can be obtained.

One aspect of the present invention is a light-emitting elementincluding a light-emitting layer, a first layer, and a second layerbetween a first electrode and a second electrode, wherein the firstlayer is provided between the light-emitting layer and the firstelectrode, the second layer is provided between the light-emitting layerand the second electrode, the first layer includes a first organiccompound and a second organic compound, in the first layer the weightpercent of the first organic compound is higher than the weight percentof the second organic compound, the first organic compound is an organiccompound having a hole-transporting property, the second organiccompound is an organic compound having a hole-trapping property, thesecond layer includes a third organic compound and a fourth organiccompound, in the second layer the weight percent of the third organiccompound is higher than the weight percent of the fourth organiccompound, the third organic compound is an organic compound having anelectron-transporting property, the fourth organic compound is anorganic compound having a hole-transporting property, and when voltageis applied to the first electrode and the second electrode so thatpotential of the first electrode is higher than potential of the secondelectrode, light emission from the light-emitting layer can be obtained.

In the above structure, the highest occupied molecular orbital level ofthe second organic compound is preferably higher than the highestoccupied molecular orbital level of the first organic compound by 0.3 eVor more.

In the above structure, the first organic compound is preferably anaromatic amine compound.

In the above structure, the thickness of the first layer is preferablyfrom 5 nm to 20 nm, inclusive.

In the above structure, the first layer and the light-emitting layer arepreferably provided to be in contact with each other.

In the above structure, the difference between the lowest unoccupiedmolecular orbital levels of the third organic compound and the fourthorganic compound is preferably less than 0.3 eV.

In the above structure, the third organic compound is preferably a metalcomplex, and the fourth organic compound is preferably an aromatic aminecompound.

In the above structure, when the magnitude of the dipole moment of thethird organic compound is P₁ and the magnitude of the dipole moment ofthe fourth organic compound is P₂, a relation of P₁/P₂≧3 or P₁/P₂≦0.33is preferably satisfied.

In the above structure, the thickness of the second layer is preferablyfrom 5 nm to 20 nm, inclusive.

In the above structure, the second layer and the light-emitting layerare preferably provided to be in contact with each other.

Moreover, the present invention includes a light-emitting device havingthe above-described light-emitting element. The light-emitting device inthis specification includes an image display device, a light-emittingdevice, or a light source (including a lighting device). Further, thefollowing are also referred to as a light-emitting device: a module inwhich a connector, for example, an FPC (flexible printed circuit), a TAB(tape automated bonding) tape, or a TCP (tape carrier package) isattached to a panel provided with a light-emitting element; a moduleprovided with a printed wiring board at the end of the TAB tape or theTCP; and a module in which an IC (integrated circuit) is directlymounted to a light-emitting element by a COG (chip on glass) method.

Further, an electronic device using the light-emitting element of thepresent invention in its display portion is also included in the presentinvention. Consequently, one feature of an electronic device of thepresent invention is to include a display portion which is provided withthe above-described light-emitting element and a control means tocontrol light emission of the light-emitting element.

In the light-emitting element of the present invention, a layer forcontrolling the carrier transport is provided; thus, a light-emittingelement with high light emission efficiency can be obtained.Furthermore, a light-emitting element with a long lifetime can beobtained.

Further, the light-emitting element of the present invention is appliedto a light-emitting device and an electronic device, whereby alight-emitting device and an electronic device with high light emissionefficiencies and reduced power consumptions can be obtained. Inaddition, a light-emitting device and an electronic device with a longlifetime can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1D are diagrams each illustrating a light-emitting elementof the present invention.

FIGS. 2A to 2D are diagrams each illustrating a light-emitting elementof the present invention.

FIGS. 3A to 3C are diagrams each illustrating a light-emitting elementof the present invention.

FIG. 4 is a diagram illustrating a light-emitting element of the presentinvention.

FIG. 5 is a diagram illustrating a light-emitting element of the presentinvention.

FIG. 6 is a diagram illustrating a light-emitting element of the presentinvention.

FIGS. 7A and 7B are diagrams illustrating a light-emitting device of thepresent invention.

FIGS. 8A and 8B are diagrams illustrating a light-emitting device of thepresent invention.

FIGS. 9A to 9D are diagrams each illustrating an electronic device ofthe present invention.

FIG. 10 is a diagram illustrating an electronic device of the presentinvention.

FIG. 11 is a diagram illustrating an electronic device of the presentinvention.

FIG. 12 is a diagram illustrating an electronic device of the presentinvention.

FIG. 13 is a diagram illustrating a lighting device of the presentinvention.

FIG. 14 is a diagram illustrating a lighting device of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The embodiment modes according to the present invention are hereinafterdescribed in detail with reference to the drawings. However, the presentinvention is not limited to the following description, and it is easilyunderstood by those skilled in the art that the mode and detail can bevariously changed without departing from the scope and spirit of thepresent invention. Therefore, the present invention is not interpretedas being limited to the following description of the embodiment modes.

Embodiment Mode 1

One mode of a light-emitting element according to the present inventionis hereinafter described with reference to FIGS. 1A to 1D. Alight-emitting element according to the present invention has a layerfor controlling the hole transport and a layer for controlling theelectron transport.

A light-emitting element of the present invention has a plurality oflayers between a pair of electrodes. The plurality of layers are acombination of layers formed of a substance having a highcarrier-injecting property and a substance having a highcarrier-transporting property which are stacked so that a light-emittingregion is formed in a region away from the electrodes, that is,recombination of carriers is performed in an area away from theelectrodes.

In this embodiment mode, a light-emitting element includes a firstelectrode 202, a second electrode 204, and an EL layer 203 providedbetween the first electrode 202 and the second electrode 204. Note thatthis embodiment mode is described below on the assumption that the firstelectrode 202 serves as an anode and the second electrode 204 serves asa cathode. That is, this embodiment mode is described below on theassumption that, when voltage is applied to the first electrode 202 andthe second electrode 204 so that potential of the first electrode 202 ishigher than potential of the second electrode 204, light emission can beobtained.

A substrate 201 is used as a support of the light-emitting element. Forthe substrate 201, glass, plastic, or the like can be used, for example.Note that other materials may also be used as long as they serve as asupport in a manufacturing process of the light-emitting element.

As for the first electrode 202, a metal, an alloy, a conductivecompound, a mixture thereof, or the like having a high work function(specifically, preferably 4.0 eV or higher) is preferably used. Forexample, indium tin oxide (ITO), indium tin oxide containing silicon orsilicon oxide, indium zinc oxide (IZO), indium oxide containing tungstenoxide and zinc oxide (IWZO), and the like can be given. A film of such aconductive metal oxide is generally formed by sputtering, but may alsobe formed by an inkjet method, a spin coating method, or the like byapplication of a sol-gel method or the like. For example, a film ofindium zinc oxide (IZO) can be formed by a sputtering method using atarget in which zinc oxide is added into indium oxide at 1 to 20 wt %.In addition, a film of indium oxide containing tungsten oxide and zincoxide (IWZO) can be formed by a sputtering method using a target inwhich tungsten oxide and zinc oxide are included in indium oxide at 0.5to 5 wt % and at 0.1 to 1 wt %, respectively. Besides, gold (Au),platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum(Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), titanium(Ti), nitride of a metal material (e.g., titanium nitride), and the likecan be given.

In a case where a layer including a composite material described belowis used as a layer in contact with the first electrode, various metals,alloys, electrically conductive compounds, or a mixture thereof can beused for the first electrode regardless of the work function. Forexample, aluminum (Al), silver (Ag), an alloy containing aluminum(AlSi), or the like can be used. Besides, any of the following materialswith a low work function can be used for the first electrode: elementsbelonging to Group 1 and Group 2 of the periodic table, that is, alkalimetals such as lithium (Li) and cesium (Cs) and alkaline earth metalssuch as magnesium (Mg), calcium (Ca), and strontium (Sr); alloys thereof(MgAg, AlLi); rare earth metals such as europium (Eu) and ytterbium(Yb); alloys thereof; and the like. A film of an alkali metal, analkaline earth metal, or an alloy thereof can be formed by a vacuumevaporation method. In addition, a film of an alloy including an alkalimetal or an alkaline earth metal can be formed by a sputtering method.Further, a film can be formed using a silver paste or the like by aninkjet method or the like.

The EL layer 203 shown in this embodiment mode includes a hole-injectinglayer 211, a hole-transporting layer 212, a layer 213 for controllingthe hole transport, a light-emitting layer 214, a layer 215 forcontrolling the electron transport, an electron-transporting layer 216,and an electron-injecting layer 217. Note that it is acceptable as longas the EL layer 203 includes a layer for controlling the carriertransport and a light-emitting layer shown in this embodiment mode.Thus, the structure of the other stacked layers is not specificallylimited. That is, there is no particular limitation on the stackedstructure of the EL layer 203, and a layer for controlling the carriertransport and a light-emitting layer shown in this embodiment mode maybe combined with a layer formed of a substance having a highelectron-transporting property, a substance having a highhole-transporting property, a substance having a high electron-injectingproperty, a substance having a high hole-injecting property, a bipolarsubstance (a substance having high electron-transporting andhole-transporting properties), or the like. For example, the EL layer203 can be formed by an appropriate combination of a hole-injectinglayer, a hole-transporting layer, a light-emitting layer, anelectron-transporting layer, an electron-injecting layer, and the like.Specific materials for each of the layers are given below.

The hole-injecting layer 211 is a layer including a substance having ahigh hole-injecting property. As a substance having a highhole-injecting property, molybdenum oxide, vanadium oxide, rutheniumoxide, tungsten oxide, manganese oxide, or the like can be used.Besides, as a low molecular organic compound, a phthalocyanine-basedcompound such as phthalocyanine (abbreviation: H₂Pc),copper(II)phthalocyanine (abbreviation: CuPc), or vanadyl phthalocyanine(VOPc); an aromatic amine compound such as4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB),4,4′-bis(N-{4-[N′-(3-methylphenyl)-N′-phenylamino]phenyl}-N-phenylamino)biphenyl(abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B),3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2), or3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1); or the like can be given.

Alternatively, for the hole-injecting layer 211, a composite material inwhich a substance having an acceptor property is mixed into a substancehaving a high hole-transporting property can be used. Note that by usinga material in which a substance having an acceptor property is mixedinto a substance having a high hole-transporting property, a materialfor forming the electrode can be selected regardless of its workfunction. In other words, besides a material with a high work function,a material with a low work function may also be used as the firstelectrode 202. A composite material of those substances can be formed byco-evaporation of a substance having a high hole-transporting propertyand a substance having an acceptor property.

Note that in this specification, “composition” refers to not only astate where two materials are simply mixed but also a state where aplurality of materials are mixed and charge is given and receivedbetween the materials.

As an organic compound used for the composite material, variouscompounds such as an aromatic amine compound, a carbazole derivative,aromatic hydrocarbon, and a high molecular compound (oligomer,dendrimer, polymer, or the like) can be used. Note that the organiccompound used for the composite material is preferably an organiccompound having a high hole-transporting property. Specifically, asubstance having a hole mobility of 10⁻⁶ cm²/Vs or higher is preferablyused. However, other substances may also be used as long as thehole-transporting properties thereof are higher than theelectron-transporting properties thereof. Examples of an organiccompound that can be used for the composite material are specificallylisted below.

As the organic compound which can be used for the composite material,for example, an aromatic amine compound such as MTDATA, TDATA, DPAB,DNTPD, DPA3B, PCzPCA1 PCzPCA2, PCzPCN1,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB orα-NPD), orN,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD); a carbazole derivative such as4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB),9-[4-(N-carbazolyl)]phenyl-10-phenylanthracene (abbreviation: CzPA), or1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene; or anaromatic hydrocarbon compound such as2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA),2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA),9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene(abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA),9,10-bis[2-(1-naphthyl)phenyl]-2-tert-butylanthracene,9,10-bis[2-(1-naphthyl)phenyl]anthracene,2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene,2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl,10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene,tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-butyl)perylene,pentacene, coronene, 4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation:DPVBi), or 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene(abbreviation: DPVPA) can be given.

As the substance having an acceptor property, an organic compound suchas 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ) or chloranil; or a transition metal oxide can be given. Inaddition, oxide of metals that belong to Group 4 to Group 8 of theperiodic table can be given as the substance having an acceptorproperty. Specifically, vanadium oxide, niobium oxide, tantalum oxide,chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, andrhenium oxide are preferable because of their high electron-acceptingproperties. Among these, molybdenum oxide is especially preferable sinceit is stable in the air and its hygroscopic property is low so that itcan be easily handled.

Further, for the hole-injecting layer 211, a high molecular compound (anoligomer, a dendrimer, a polymer, or the like) can be used. For example,a high molecular compound such as poly(N-vinylcarbazole) (abbreviation:PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTTA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), orpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine (abbreviation:Poly-TPD) can be given. In addition, a high molecular compound to whichacid is added. As an example of acid,poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT/PSS),polyaniline/poly(styrenesulfonic acid) (PAni/PSS), or the like can beused.

Further, a composite material formed by using the above-mentioned highmolecular compound such as PVK, PVTPA, PTPDMA, or Poly-TPD and theabove-mentioned substance having an acceptor property can be used forthe hole-injecting layer 211.

The hole-transporting layer 212 is a layer including a substance havinga high hole-transporting property. As a substance having a highhole-transporting property, an aromatic amine compound such as NPB (orα-NPD), TPD,4,4′-bis[N-(9,9-dimethylfluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: DFLDPBi),4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), or the like can be given as the low molecularorganic compound. The substances mentioned here mainly have a holemobility of 10⁻⁶ cm²/Vs or higher. However, other substances may also beused as long as the hole-transporting properties thereof are higher thanthe electron-transporting properties thereof. Note that the layerincluding a substance having a high hole-transporting property is notlimited to a single layer, but two or more layers including any of theabove-mentioned substances may be stacked.

Further, for the hole-transporting layer 212, a high molecular compoundsuch as PVK, PVTPA, PTPDMA, or Poly-TPD can be used.

The layer 213 for controlling the hole transport shown in thisembodiment mode includes the first organic compound and the secondorganic compound, in which the weight percent of the first organiccompound is higher than that of the second organic compound. That is,the second organic compound is dispersed into the first organiccompound. The layer 213 for controlling the hole transport is preferablyprovided between the light-emitting layer 214 and the first electrode202.

In a case where the layer for controlling the hole transport isprovided, the first organic compound is preferably an organic compoundhaving a hole-transporting property. That is, the first organic compoundis preferably a substance whose hole-transporting property is higherthan the electron-transporting property.

On the other hand, the second organic compound is preferably an organiccompound having a function of trapping holes. That is, the secondorganic compound is preferably an organic compound whose highestoccupied molecular orbital (HOMO) level is higher than that of the firstorganic compound by 0.3 eV or more.

Since the second organic compound is included, the hole-transportingrate of the layer for controlling the hole transport as a whole is lowerthan that of a layer including only the first organic compound. That is,by adding the second organic compound, the carrier transport can becontrolled. Further, by control of the concentration of the secondorganic compound, the carrier-transporting rate can be controlled.Specifically, the concentration of the second organic compound ispreferably from 0.1 wt % to 5 wt % or from 0.1 mol % to 5 mol %.

FIG. 4 exemplarily illustrates a band diagram of a light-emittingelement of the present invention in FIG. 1A. In FIG. 4, electronsinjected from the second electrode 204 pass through theelectron-injecting layer 217 and the electron-transporting layer 216,and are injected into the layer 215 for controlling the electrontransport. On the other hand, the holes injected from the firstelectrode 202 pass through the hole-injecting layer 211 and thehole-transporting layer 212, and are injected into the layer 213 forcontrolling the hole transport. The transport of the holes injected intothe layer for controlling the hole transport is retarded by the secondorganic compound having a hole-trapping property. The holes of whichtransport is retarded are injected into the light-emitting layer 214,and then recombined with holes. Thus, light emission is obtained.

As the second organic compound, for example, CuPc, DNTPD, DPAB,bis(2-phenylpyridinato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(Ppy)₂(acac)),(acetylacetonato)bis[10-(2-pyridyl)phenoxazinato]iridium(III)(abbreviation: Ir(ppx)₂(acac)),tris(2-phenylpyridinato-N,C^(2′))iridium(III), (abbreviation: Ir(ppy)₃),bis[2-(2′-benzo[4,5-α]thienyl)pyridinato-N,C³′]iridium(III)acetylacetonate(abbreviation: Ir(btp)₂(acac)),N,N″-(2-tert-butylanthracene-9,10-diyldi-4,1-phenylene)bis[N,N′,N′-triphenyl-1,4-phenylenediamine](abbreviation: DPABPA), or the like can be given.

The above-described compounds are compounds having particularly highHOMO levels among compounds that are used for light-emitting elements.Thus, when such compounds are added to the first organic compound whichwill be described later, an excellent hole-trapping property isexhibited. Note that it is preferable that emission colors of thelight-emitting layer and the second organic compound be similar colors.In this manner, even if the second organic compound unintendedly emitslight, the color purity of the light-emitting elements can be kept.

In addition, the first organic compound included in the layer 213 forcontrolling the hole transport is an organic compound having ahole-transporting property. That is, the first organic compound is asubstance whose hole-transporting property is higher than theelectron-transporting property. Specifically, a condensed aromatichydrocarbon such as 9,10-diphenylanthracene (abbreviation: DPAnth) or6,12-dimethoxy-5,11-diphenylchrysene can be given.

In addition, an aromatic amine compound such asN,N-dipheyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: CzA1PA), 4-(10-phenyl-9-anthryl)triphenylamine(abbreviation: DPHPA),N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole-3-amine(abbreviation: PCAPA),N,9-diphenyl-N-{4-[4-(10-phenyl-9-anthryl)phenyl]phenyl}-9H-carbazol-3-amine](abbreviation: PCAPBA),N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine,(abbreviation: 2PCAPA), NPB (or α-NPD), TPD, DFLDPBi, BSPB, or2,3-bis{4-[N-(4-biphenylyl)-N-phenylamino]phenyl}quinoxaline(abbreviation: BPAPQ) can be given. Further, a high molecular compoundsuch as PVK, PVTPA, PTPDMA, or Poly-TPD can be used.

Above all, an aromatic amine compound that is stable against holes ispreferably used. In addition, as described above, in this embodimentmode, the second organic compound should be a compound having ahole-trapping property. Therefore, the HOMO level of the second organiccompound is preferably higher than that of the first organic compound by0.3 eV or more. Therefore, it is acceptable as long as the first organiccompound is appropriately selected according to the kind of compoundused for the second organic compound so as to satisfy the abovecondition.

Note that it is preferable that the emission colors of the secondorganic compound included in the layer 213 for controlling the holetransport and a substance having a high light-emitting property includedin the light-emitting layer 214 be similar colors. Specifically, it ispreferable that the difference between the wavelength of the maximumpeak of the emission spectrum of the second organic compound and that ofthe substance having a high light-emitting property be 30 nm or less.When the difference between the wavelength of the maximum peak of theemission spectrum of the second organic compound and that of thesubstance having a high light-emitting property is 30 nm or less, theemission colors of the substance having a high light-emitting propertyand the second organic compound can be similar colors. Accordingly, evenin a case where the second organic compound emits light due to change ofvoltage or the like, change in emission color can be suppressed.

However, the second organic compound has no necessity to emit light. Forexample, in a case where light emission efficiency of the substancehaving a high light-emitting property is higher than that of the secondorganic compound, the concentration of the second organic compound inthe layer 213 for controlling the hole transport is preferably adjusted(the concentration is slightly lowered so that light emission from thesecond organic compound is suppressed) so that only light emitted fromthe substance having a high light-emitting property is substantiallyobtained. In that case, the emission colors of the substance having ahigh light-emitting property and the second organic compound are similarcolors (i.e., they have about the same level of energy gap). Therefore,there is little possibility that energy will transfer from the substancehaving a high light-emitting property toward the second organiccompound, and thus high light emission efficiency can be obtained.

In addition, the thickness of the layer 213 for controlling the holetransport is preferably from 5 nm to 20 nm, inclusive. When thethickness is too large, the carrier-transporting rate becomes too slow,which could result in high driving voltage. When the thickness is toosmall, on the other hand, it is impossible to implement the function ofcontrolling the carrier transport. Therefore, the thickness ispreferably from 5 nm to 20 nm, inclusive.

In a case of a conventional element where a layer for controlling thehole transport is not provided, holes injected from the first electrodepass through a hole-injecting layer and a hole-transporting layer to beinjected into a light-emitting layer. If the light-emitting layer has ahole-transporting property, that is, if the material which has thehighest weight percent in the light-emitting layer has ahole-transporting property, holes injected into the light-emitting layertransfer through the light-emitting layer, and may reach anelectron-transporting layer. When holes reach the electron-transportinglayer, materials which are included in the electron-transporting layerare degraded, leading deterioration of the light-emitting element.

However, by providing the layer for controlling the hole transport shownin this embodiment mode, holes that penetrate the light-emitting layerand reach the electron-transporting layer can be suppressed. Therefore,deterioration of the electron-transporting layer, which is causedbecause holes reach the electron-transporting layer, can be suppressed.Thus, deterioration of the light-emitting element can be prevented, andthe light-emitting element with a long lifetime can be obtained.

On the other hand, in a case of a conventional element where the layerfor controlling the hole transport is not provided, most of the holesinjected from the first electrode are injected into the light-emittinglayer without the transport being controlled. If the light-emittinglayer has an electron-transporting property, that is, if the materialwhich has the highest weight percent in the light-emitting layer has anelectron-transporting property, a light-emitting region is formed in thevicinity of the interface between the light-emitting layer and thehole-transporting layer. In addition, there is a possibility thatcations are generated by excessive holes in the vicinity of theinterface between the light-emitting layer and the hole-transportinglayer. Since a cation serves as a quencher, light emission efficiencydecreases due to cations generated in the vicinity of the light-emittingregion.

However, by providing the layer for controlling the hole transport shownin this embodiment mode, formation of cations generated by excessiveholes in the light-emitting layer and in the vicinity of thelight-emitting layer can be suppressed, and decrease in light emissionefficiency can be suppressed. Therefore, a light-emitting element withhigh light emission efficiency can be obtained.

As described above, by controlling the hole transport, the carrierbalance is improved. As a result, the recombination probability of holesand electrons is improved, thereby high light emission efficiency can beobtained. Note that as shown in this embodiment mode, a structure inwhich the layer for controlling the hole transport is provided betweenthe light-emitting layer and the first electrode serving as an anode isparticularly effective for a light-emitting element having excessiveholes. This is because the transport of excessive holes can besuppressed and controlled so that the balance of holes and electrons canbe achieved by providing the layer for controlling the hole transport inthe light-emitting element having excessive holes.

The light-emitting layer 214 is a layer including a substance having ahigh light-emitting property, and various materials can be used for thelight-emitting layer. For example, as a substance having a highlight-emitting property, a fluorescent compound which emits fluorescenceor a phosphorescent compound which emits phosphorescence can be used.

Examples of a phosphorescent compound which is used for thelight-emitting layer are given below. As a material for bluish lightemission,bis[2-(4′,6′-difluorophenyl)pyridinato-N,C²′]iridium(III)tetrakis(1-pyrazolyl)borate(abbreviation: FIr6),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C²′]iridium(III)picolinate(abbreviation: FIrpic),bis[2-(3′,5′bistrifluoromethylphenyl)pyridinato-N,C²′]iridium(III)picolinate(abbreviation: Ir(CF₃ppy)₂(pic)),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C²′]iridium(III)acetylacetonate(abbreviation: FIr(acac)) or the like can be given. As a material forgreenish light emission, tris(2-phenylpyridinato-N,C^(2′))iridium(III)(abbreviation: Ir(ppy)₃),bis[2-phenylpyridinato-N,C²′]iridium(III)acetylacetonate (abbreviation:Ir(ppy)₂(acac)),bis(1,2-diphenyl-1H-benzimidazolato)iridium(III)acetylacetonate(abbreviation: Ir(pbi)₂(acac)),bis(benzo[h]quinolinato)iridium(III)acetylacetonate (abbreviation:Ir(bzq)₂(acac)) or the like can be given. As a material for yellowishlight emission,bis(2,4-diphenyl-1,3-oxazolato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(dpo)₂(acac)),bis[2-(4′-perfluorophenylphenyl)pyridinato]iridium(III)acetylacetonate(abbreviation: Ir(p-PF-ph)₂(acac)),bis(2-phenylbenzothiazolato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(bt)₂(acac)) or the like can be given. As a materialfor orangish light emission, tris(2-phenylquinolinato-N,C²′)iridium(III)(abbreviation: Ir(pq)₃),bis(2-phenylquinolinato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(pq)₂(acac)) or the like can be given. As a materialfor reddish light emission, organometallic complex such asbis[2-(2′-benzo[4,5-α]thienyl)pyridinato-N,C³′]iridium(III)acetylacetonate(abbreviation: Ir(btp)₂(acac)),bis(1-phenylisoquinolinato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(piq)₂(acac)),(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)), or(2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrinato)platinum(II)(abbreviation: PtOEP) can be given. In addition, a rare earth metalcomplex such as tris(acetylacetonato)(monophenanthroline)terbium(III)(abbreviation: Tb(acac)₃(Phen)),tris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III)(abbreviation: Eu(DBM)₃(Phen)), ortris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III)(abbreviation: Eu(TTA)₃(Phen)) performs light emission (electrontransition between different multiplicities) from a rare earth metalion; therefore, such a rare earth metal complex can be used as aphosphorescent compound.

Examples of a fluorescent compound which is used for the light-emittinglayer are given below. As a material for bluish light emission,N,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA), or the like can be given. As a material forgreenish light emission,N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation, 2PCAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCABPhA),N-(9,10-diphenyl-2-anthryl)-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPABPhA),9,10-di(2-biphenylyl)-2-{N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylamino}anthracene(abbreviation: 2YGABPhA), N,N,9-triphenylanthracen-9-amine(abbreviation: DPhAPhA), or the like can be given. As a material foryellowish light emission, rubrene,5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT),or the like can be given. As a material for reddish light emission,N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation:p-mPhTD),7,13-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-α]fluoranthene-3,10-diamine(abbreviation: p-mPhAFD), or the like can be given.

Note that the light-emitting layer may have a structure in which any ofthe above substances having a high light-emitting property (a sixthorganic compound) is dispersed into another substance (a fifth organiccompound). As the substance into which the substance having a highlight-emitting property is dispersed, various kinds of materials can beused, and it is preferable to use a substance whose lowest unoccupiedmolecular orbital (LUMO) level is higher than that of a substance havinga high light-emitting property and whose highest occupied molecularorbital (HOMO) level is lower than that of the substance having a highlight-emitting property.

As the substance into which the substance having a high light-emittingproperty is dispersed, specifically, a metal complex such astris(8-quinolinolato)aluminum(III) (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), orbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: Zn(BTZ)₂); aheterocyclic compound such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ01),2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), bathophenanthroline (abbreviation: BPhen), orbathocuproine (BCP); a condensed aromatic compound such as9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA),3,6-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: DPCzPA), 9,10-bis(3,5-diphenylphenyl)anthracene(abbreviation: DPPA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA),2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),9,9′-bianthryl (abbreviation: BANT),9,9′-(stilbene-3,3′-diyl)diphenanthrene (abbreviation: DPNS),9,9′-(stilbene-4,4′-diyl)diphenanthrene (abbreviation: DPNS2),3,3′,3″-(benzene-1,3,5-triyl)tripyrene (abbreviation: TPB3),9,10-diphenylanthracene (abbreviation: DPAnth), or6,12-dimethoxy-5,11-diphenylchrysene; an aromatic amine compound such asN,N-dipheyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: CzA1PA), 4-(10-phenyl-9-anthryl)triphenylamine(abbreviation: DPhPA),N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole-3-amine(abbreviation: PCAPA),N,9-diphenyl-N-{4-[4-(10-phenyl-9-anthryl)phenyl]phenyl}-9H-carbazol-3-amine(abbreviation: PCAPBA),N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA), NPB (or α-NPD), TPD, DFLDPBi, or BSPB; or thelike can be used.

As the substance into which the substance having a high light-emittingproperty is dispersed, a plurality of kinds of substances can be used.For example, in order to suppress crystallization, a substance forsuppressing crystallization such as rubrene or the like may be furtheradded. Furthermore, in order to efficiently transfer energy to thesubstance having a light-emitting property, NPB, Alq, or the like may befurther added.

When a structure in which the substance having a high light-emittingproperty is dispersed into another substance is employed,crystallization of the light-emitting layer 214 can be suppressed.Further, concentration quenching due to high concentration of thesubstance having a high light-emitting property can be suppressed.

Note that for the light-emitting layer 214, a high molecular compoundcan be used. Specifically, as a material for bluish light emission,poly(9,9-dioctylfluorene-2,7-diyl) (abbreviation: POF),poly[(9,9-dioctylfluorene-2,7-diyl-co-(2,5-dimethoxybenzene-1,4-diyl)](abbreviation: PF-DMOP),poly{(9,9-dioctylfluorene-2,7-diyl)-co-[N,N′-di-(p-butylphenyl)-1,4-diaminobenzene]}(abbreviation: TAB-PFH) or the like can be given. As a material forgreenish light emission, poly(p-phenylenevinylene) (abbreviation: PPV),poly[(9,9-dihexylfluorene-2,7-diyl)-alt-co-(benzo[2,1,3]thiadiazole-4,7-diyl)](abbreviation: PFBT),poly[(9,9-dioctyl-2,7-divinylenefluorenylene)-alt-co-(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene)],or the like can be given. As a material for orangish to reddish lightemission, poly[2-methoxy-5-(2′-ethylhexoxy)-1,4-phenylenevinylene](abbreviation: MEH-PPV), poly(3-butylthiophene-2,5-diyl) (abbreviation:R4-PAT),poly{[9,9-dihexyl-2,7-bis(1-cyanovinylene)fluorenylene]-alt-co-[2,5-bis(N,N′-diphenylamino)-1,4-phenylene]},poly{[2-methoxy-5-(2-ethylhexyloxy)-1,4-bis(1-cyanovinylenephenylene)]-alt-co-[2,5-bis(N,N′-diphenylamino)-1,4-phenylene]}(abbreviation: CN-PPV-DPD) or the like can be given.

The layer 215 for controlling the electron transport includes a thirdorganic compound and a fourth organic compound, in which the weightpercent of the third organic compound is higher than that of the fourthorganic compound. That is, the fourth organic compound is dispersed intothe third organic compound. The layer 215 for controlling the electrontransport is preferably provided between the light-emitting layer 214and the second electrode 204.

The layer 215 for controlling the electron transport shown in thisembodiment mode includes the third organic compound and the fourthorganic compound, and the third organic compound and the fourth organiccompound transport different kinds of carriers.

In a case where the layer for controlling the electron transport isprovided between the light-emitting layer and the second electrodeserving as a cathode, the third organic compound is preferably anorganic compound having an electron-transporting property, and thefourth organic compound is preferably an organic compound having ahole-transporting property. That is, the third organic compound ispreferably a substance whose electron-transporting property is higherthan the hole-transporting property, while the fourth organic compoundis preferably a substance whose hole-transporting property is higherthan the electron-transporting property. In addition, the differencebetween the lowest unoccupied molecular orbital (LUMO) levels of thethird organic compound and the fourth organic compound is preferablyless than 0.3 eV, and more preferably 0.2 eV or less. That is, it ispreferable that, in thermodynamic terms, electrons, which are carriers,can be easily transported between the third organic compound and thefourth organic compound.

FIG. 5 illustrates a conceptual view of a layer for controlling theelectron transport shown in this embodiment mode. In FIG. 5, since athird organic compound 241 has an electron-transporting property,electrons are easily injected thereinto and transported to neighboringthird organic compound. That is, the rate at which electrons areinjected into the third organic compound and the rate (ν) at which theelectrons are released from the third organic compound are high.

Meanwhile, in thermodynamic terms, there is a possibility that electronsare injected into a fourth organic compound 242 which is an organiccompound having a hole-transporting property because the LUMO level ofthe fourth organic compound 242 is close to that of the third organiccompound 241. However, the rate (ν₁) at which electrons are injectedfrom the third organic compound 241, which is an organic compound havingan electron-transporting property, into the fourth organic compound 242,which is an organic compound having a hole-transporting property, or therate (ν₂), at which electrons are injected from the fourth organiccompound 242 into the third organic compound 241, is lower than the rate(ν) at which electrons are injected from the third organic compound 241into another third organic compound 241.

Since the fourth organic compound 242 is included, theelectron-transporting rate of the layer for controlling the electrontransport as a whole is lower than that of a layer including only thethird organic compound 241. That is, by adding the fourth organiccompound 242, the carrier transport can be controlled. Further, bycontrolling the concentration of the fourth organic compound 242, thecarrier-transporting rate can be controlled.

As described above, the third organic compound is preferably an organiccompound having an electron-transporting property in this embodimentmode. Specifically, a metal complex such astris(8-quinolinolato)aluminum(III) (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), orbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: Zn(BTZ)₂), canbe used. Further, as an alternative to such a metal complex, aheterocyclic compound such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ01),2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), bathophenanthroline (abbreviation: BPhen), orbathocuproine (abbreviation: BCP) can be used. Further, a condensedaromatic compound such as 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: CzPA),3,6-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: DPCzPA), 9,10-bis(3,5-diphenylphenyl)anthracene(abbreviation: DPPA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA),2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),9,9′-bianthryl (abbreviation: BANT),9,9′-(stilbene-3,3′-diyl)diphenanthrene (abbreviation: DPNS),9,9′-(stilbene-4,4′-diyl)diphenanthrene (abbreviation: DPNS2), or3,3′,3″-(benzene-1,3,5-triyl)tripyrene (abbreviation: TPB3) can be used.Further, a high molecular compound such aspoly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation: PF-Py), orpoly[(9,9-dioctyllfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation: PF-BPy) can be used.

The fourth organic compound is preferably an organic compound having ahole-transporting property. Specifically, a condensed aromatichydrocarbon such as 9,10-diphenylanthracene (abbreviation: DPAnth) or6,12-dimethoxy-5,11-diphenylchrysene, an aromatic amine compound such asN,N-dipheyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: CzA1PA), 4-(10-phenyl-9-anthryl)triphenylamine(abbreviation: DPhPA),N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA),N,9-diphenyl-N-[4-[4-(10-phenyl-9-anthryl)phenyl]phenyl]-9H-carbazol-3-amine(abbreviation: PCAPBA),N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine,(abbreviation: 2PCAPA), NPB (or α-NPD), TPD, DFLDPBi, or BSPB, acompound including an amino group such as Coumarin 7, or Coumarin 30,and the like can be used. Further, a high molecular compound such asPVK, PVTPA, PThDMA, or Poly-TPD can be used.

By the above combination, the electron transport from the third organiccompound to the fourth organic compound or from the fourth organiccompound to the third organic compound is suppressed, whereby theelectron-transporting rate in the layer for controlling the carriertransport can be suppressed. Further, the layer for controlling thecarrier transport has a structure in which the fourth organic compoundis dispersed into the third organic compound; therefore, crystallizationor aggregation is hardly caused with time. Accordingly, theabove-described effect of suppressing the electron transport is hardlychanged with time, and as a result, the carrier balance hardly changeswith time. This leads to improvement in lifetime, in other words,improvement in reliability of the light-emitting element.

Note that among the above-described combinations, a metal complex as thethird organic compound and an aromatic amine compound as the fourthorganic compound are preferably combined. A metal complex has a highelectron-transporting property and a large dipole moment, whereas anaromatic amine compound has a high hole-transporting property and acomparatively small dipole moment. In such a manner, by combination ofsubstances dipole moments of which are largely different from eachother, the above-described effect of suppressing the electron transportbecomes more significant. Specifically, when the magnitude of the dipolemoment of the third organic compound is P₁ and the magnitude of thedipole moment of the fourth organic compound is P₂, a combination whichsatisfies P₁/P₂≧3 or P₁/P₂≦0.33 is preferable.

For example, the dipole moment of Alq that is a metal complex is 9.40debye, and the dipole moment of 2PCAPA that is an aromatic aminecompound is 1.15 debye. Accordingly, as in this embodiment mode, when anorganic compound having an electron-transporting property like a metalcomplex is used as the third organic compound and an organic compoundhaving a hole-transporting property like an aromatic amine compound isused as the fourth organic compound, P₁/P₂≧3 is preferably satisfied.

In addition, it is preferable that emission colors of the fourth organiccompound included in the layer 215 for controlling the electrontransport and the substance having a high light-emitting propertyincluded in the light-emitting layer 214 be similar colors.Specifically, it is preferable that the difference between thewavelength of the maximum peak of the emission spectrum of the fourthorganic compound and that of the substance having a high light-emittingproperty be 30 nm or less. When the difference between the wavelength ofthe maximum peak of the emission spectrum of the fourth organic compoundand that of the substance having a high light-emitting property is 30 nmor less, the emission colors of the fourth organic compound and thesubstance having a high light-emitting property can be similar colors.Accordingly, even in a case where the fourth organic compound emitslight due to change in voltage or the like, change in emission color canbe suppressed. However, the fourth organic compound has no necessity toemit light.

In addition, the thickness of the layer 215 for controlling the electrontransport is preferably from 5 nm to 20 nm, inclusive. When the layer215 for controlling the electron transport is too thick, thecarrier-transporting rate becomes too slow, which could result in highdriving voltage, in addition, the emission intensity of the layer 215for controlling the electron transport may increase. When the layer 215for controlling the carrier transport is too thin, on the other hand, itis impossible to implement the function of controlling the carriertransport. Therefore, the thickness is preferably from 5 nm to 20 nm,inclusive.

On the other hand, in a case of a conventional element where the layerfor controlling the electron transport is not provided, electronsinjected from the second electrode pass through an electron-injectinglayer and an electron-transporting layer to be injected into thelight-emitting layer. In a case where the light-emitting layer is alayer having an electron-transporting property, that is, in a case wherethe material which has the highest weight percent in the light-emittinglayer has an electron-transporting property, electrons injected into thelight-emitting layer transfer through the light-emitting layer, and mayreach a hole-transporting layer. When electrons reach thehole-transporting layer, materials which are included in thehole-transporting layer are degraded, leading deterioration of thelight-emitting element.

However, by providing the layer for controlling the electron transportshown in this embodiment mode, electrons that penetrate thelight-emitting layer and reach the hole-transporting layer can besuppressed. Therefore, deterioration of the hole-transporting layer,which is caused because electrons reach the hole-transporting layer, canbe suppressed. Thus, deterioration of the light-emitting element can besuppressed, and the light-emitting element with a long lifetime can beobtained.

The electron-transporting layer 216 is a layer including a substancehaving a high electron-transporting property. For example, as a lowmolecular organic compound, a metal complex such astris(8-quinolinolato)aluminum(III) (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZNPBO), orbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: Zn(BTZ)₂), canbe used. Further, as an alternative to such a metal complex, aheterocyclic compound such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ01),2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), bathophenanthroline (abbreviation: BPhen), orbathocuproine (abbreviation: BCP) can be used. The substances mentionedhere mainly have a hole mobility of 10⁻⁶ cm²/Vs or higher. However,other substances than the substances mentioned above may also be usedfor the electron-transporting layer as long as the electron-transportingproperties thereof are higher than the hole-transporting propertiesthereof. Note that the electron-transporting layer is not limited to asingle layer, but two or more layers including the above-mentionedsubstances may be stacked.

Further, for the electron-transporting layer 216, a high molecularcompound can be used. For example,poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation: PF-Py),poly[(9,9-dioctyllfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation: PF-BPy), or the like can be used.

The electron-injecting layer 217 is a layer including a substance havinga high electron-injecting property. As a substance having a highelectron-injecting property, an alkali metal, an alkaline earth metal,or a compound thereof such as lithium fluoride (LiF), cesium fluoride(CsF), or calcium fluoride (CaF₂) can be used. For example, a layer of asubstance having an electron-transporting property which furtherincludes an alkali metal, an alkaline earth metal, or a compoundthereof; for example, a layer of Alq including magnesium (Mg) can beused. Note that it is preferable to use the layer formed of a substancehaving an electron-transporting property in which an alkali metal or analkaline earth metal is mixed as the electron-injecting layer becauseelectrons can be efficiently injected from the second electrode 204.

As a substance for forming the second electrode 204, a metal, an alloy,an electrically conductive compound, or a mixture thereof, or the likewith a low work function (specifically, a work function of 3.8 eV orlower is preferable) can be used. Specific examples of such cathodematerials are given below: elements belonging to Group 1 and Group 2 ofthe periodic table, that is, alkali metals such as lithium (Li) andcesium (Cs) and alkaline earth metals such as magnesium (Mg), calcium(Ca), and strontium (Sr); alloys thereof (MgAg, AlLi); rare earth metalssuch as europium (Eu) and ytterbium (Yb); alloys thereof; and the like.A film of an alkali metal, an alkaline earth metal, or an alloy thereofcan be formed by a vacuum evaporation method. In addition, a film of analloy including an alkali metal or an alkaline earth metal can be formedby a sputtering method. Further, a film can be formed using a silverpaste or the like by an inkjet method or the like.

By providing the electron-injecting layer 217 which is a layer having afunction of promoting electron injection between the second electrode204 and the electron-transporting layer 216, the second electrode 204can be formed using various conductive materials such as Al, Ag, ITO, orindium tin oxide containing silicon or silicon oxide, regardless oftheir work functions. A film of such a conductive material can be formedby a sputtering method, an inkjet method, a spin coating method, or thelike.

Various methods can be used for forming an EL layer, regardless of a drymethod or a wet method. For example, a vacuum evaporation method, aninkjet method, a spin coating method, or the like may be used. Further,a different film formation method may be used to form each electrode oreach layer.

For example, the EL layer may be formed by a wet method using a highmolecular compound selected from the above-mentioned materials. Further,the EL layer can also be formed by a wet method using a low molecularorganic compound. Furthermore, the EL layer may be formed by a drymethod such as a vacuum evaporation method using a low molecular organiccompound.

The electrode may be formed by a wet method using a sol-gel method, orby a wet method using a paste of a metal material. Further, theelectrode may be formed by a dry method such as a sputtering method or avacuum evaporation method.

For example, in a case where a light-emitting element of the presentinvention is applied to a display device and a light-emitting layer foreach color is formed separately, it is preferable to form thelight-emitting layer by a wet method. When the light-emitting layer isformed by an inkjet method, it becomes easy to form the light-emittinglayers separately for each color even when a large-sized substrate isused.

In a light-emitting element of the present invention having theabove-described structure, current flows due to a potential differencegenerated between the first electrode 202 and the second electrode 204,whereby holes and electrons are recombined in the EL layer 203 and lightemission is obtained.

Light emission is extracted to the outside through one of or both thefirst electrode 202 and the second electrode 204. Accordingly, one of orboth the first electrode 202 and the second electrode 204 are electrodeshaving a light-transmitting property. When only the first electrode 202has a light-transmitting property, light emission is extracted from thesubstrate side through the first electrode 202 as illustrated in FIG.3A. When only the second electrode 204 has a light-transmittingproperty, light emission is extracted from the opposite side to thesubstrate through the second electrode 204 as illustrated in FIG. 3B.When both the first electrode 202 and the second electrode 204 have alight-transmitting property, light emission is extracted from both thesubstrate side and the opposite side to the substrate through the firstelectrode 202 and the second electrode 204 as illustrated in FIG. 3C.

Note that the structure of the layers provided between the firstelectrode 202 and the second electrode 204 is not limited to the abovestructure. Any structure other than the above structure can be employedas long as a light-emitting region for recombination of holes andelectrons is positioned away from the first electrode 202 and the secondelectrode 204 so as to prevent quenching caused by proximity of thelight-emitting region to metal, and a layer for controlling the carriertransport is provided.

That is, there is no particular limitation on the stacked structure ofthe layers, and a layer for controlling the carrier transport and thelight-emitting layer shown in this embodiment mode may be combined witha layer formed of a substance having a high electron-transportingproperty, a substance having a high hole-transporting property, asubstance having a high electron-injecting property, a substance havinga high hole-injecting property, a bipolar substance (a substance havinghigh electron-transporting and hole-transporting properties), or thelike.

Note that since the layer for controlling the hole transport controlsthe hole transport, the layer for controlling the hole transport ispreferably provided between the light-emitting layer and the electrodeserving as an anode.

As illustrated in FIGS. 1A and 1C, in a case of a structure where thelight-emitting layer 214 and the layer 213 for controlling the holetransport are in contact with each other, as the second organiccompound, it is preferable to use an organic compound into whichelectrons are not easily injected and the band gap thereof is higherthan that of an organic compound which has the highest weight percent inthe light-emitting layer 214. In a case of a structure where thelight-emitting layer 214 and the layer 213 for controlling the holetransport are in contact with each other, the light-emitting layer andthe layer for controlling the carrier transport can be successivelyformed with the same mask which is preferable in manufacturing afull-color display or the like where selective formation of the layerfor controlling the carrier transport is needed for each light-emittingelement because the manufacture is facilitated.

However, as illustrated in FIGS. 1B and 1D, a structure in which a layeris formed between the light-emitting layer 214 and the layer 213 forcontrolling the hole transport may be employed.

Note that since the layer for controlling the electron transportcontrols the electron transport, the layer for controlling the electrontransport is preferably provided between the light-emitting layer andthe electrode serving as a cathode. As illustrated in FIGS. 1A and 1B, alayer for controlling the electron transport is more preferably providedto be in contact with the light-emitting layer. By providing the layerfor controlling the electron transport to be in contact with thelight-emitting layer, electron injection into the light-emitting layercan be directly controlled. Therefore, change in carrier balance withtime in the light-emitting layer can be controlled more efficiently,whereby the lifetime of the element can be more effectively improved. Inaddition, the process can be simplified.

Note that the layer for controlling the electron transport is preferablyprovided to be in contact with the light-emitting layer, and in such acase, it is preferable that the third organic compound included in thelayer for controlling the electron transport be different from anorganic compound which has high weight percent in the light-emittinglayer. In particular, in a case where the light-emitting layer includesa substance (a fifth organic compound) for dispersing a substance havinga high light-emitting property and a substance having a highlight-emitting property (a sixth organic compound), the kinds of thefifth organic compound and the third organic compound are preferablydifferent from each other. In such a structure, the electron transportfrom the layer for controlling the electron transport to thelight-emitting layer is suppressed also between the third organiccompound and the fifth organic compound, and thus effect of providingthe layer for controlling the electron transport is further increased.

However, as illustrated in FIGS. 1C and 1D, a layer may be formedbetween the light-emitting layer 214 and the layer 215 for controllingthe electron transport.

In addition, as illustrated in FIGS. 2A to 2D, a structure in which thesecond electrode 204 serving as a cathode, the EL layer 203, and thefirst electrode 202 serving as an anode are stacked sequentially overthe substrate 201 may be employed. The light-emitting element in FIG. 2Ahas a structure in which the layers of the EL layer in FIG. 1A arestacked in the reverse order, the light-emitting element in FIG. 2B hasa structure in which the layers of the EL layer in FIG. 1B are stackedin the reverse order, the light-emitting element in FIG. 2C has astructure in which the layers of the EL layer in FIG. 1C are stacked inthe reverse order, and the light-emitting element in FIG. 2D has astructure in which the layers of the EL layer in FIG. 1D are stacked inthe reverse order.

Note that in this embodiment mode, the light-emitting element is formedover a substrate made of glass, plastic, or the like. By forming aplurality of such light-emitting elements over a substrate, a passivematrix light-emitting device can be manufactured. Moreover, for example,thin film transistors (TFTs) may be formed over a substrate made ofglass, plastic, or the like so that light-emitting elements aremanufactured over electrodes which are electrically connected to theTFTs. Thus, an active matrix light-emitting device which controls thedriving of a light-emitting element by a TFT can be manufactured. Notethat a structure of the TFT is not particularly limited, and either astaggered TFT or an inverted staggered TFT may be used. In addition, adriving circuit formed over a TFT substrate may be formed using anN-channel TFT and a P-channel TFT, or may be formed using either anN-channel TFT or a P-channel TFT. In addition, the crystallinity of asemiconductor film used for the TFT is not particularly limited. Eitheran amorphous semiconductor film or a crystalline semiconductor film maybe used for the TFT. Further, a single crystal semiconductor film may beused. A single crystal semiconductor film can be formed by a Smart Cut(registered trademark) method or the like.

As described above, the light-emitting element shown in this embodimentmode is characterized by having both the layer 213 for controlling thehole transport and the layer 215 for controlling the electron transport.

For example, in a case of a conventional light-emitting element wherethe layer 213 for controlling the hole transport and the layer 215 forcontrolling the electron transport are not provided, holes injected fromthe first electrode 202 pass through the hole-injecting layer 211 andthe hole-transporting layer 212 to be injected into the light-emittinglayer 214 without the transport being retarded; therefore, part of theholes reach the vicinity of the interface between the light-emittinglayer and the electron-transporting layer 216. Thus, holes may reach theelectron-transporting layer 216 and deteriorate theelectron-transporting layer 216. Due to the deterioration, when thenumber of holes which reach the electron-transporting layer 216 isincreased with time, the recombination probability in the light-emittinglayer 214 is reduced with time, which results in reduction in elementlifetime (luminance decay with time). Similarly, the electrons injectedfrom the second electrode 204 pass through the electron-injecting layer217 and the electron-transporting layer 216 to be injected into thelight-emitting layer 214 without the transport being retarded;therefore, part of the electrons reach the vicinity of the interfacebetween the hole-transporting layer 212 and the light-emitting layer214. Thus, electrons may reach the hole-transporting layer 212 anddeteriorate the hole-transporting layer 212. Due to the deterioration,when the number of electrons which reach the hole-transporting layer 212is increased with time, the recombination probability in thelight-emitting layer 214 is reduced with time, which results inreduction in element lifetime (luminance decay with time).

On the other hand, as for a light-emitting element of the presentinvention, by providing the layer 213 for controlling the holetransport, holes injected from the first electrode 202 pass through thehole-injecting layer 211 and the hole-transporting layer 212 to beinjected into the layer 213 for controlling the hole transport. The rateof the holes injected into the layer 213 for controlling the holetransport is retarded, and hole injection into the light-emitting layer214 is controlled. As a result, the possibility that holes may reach anddeteriorate the hole-transporting layer 216 is lowered. Note that it isimportant in the present invention that an organic compound whichreduces a hole-transporting property is added to an organic compoundhaving a hole-transporting property, instead of just applying asubstance with low hole mobility in the layer 213 for controlling thehole transport. When such a structure is employed, in addition to justcontrolling hole injection into the light-emitting layer, change in thequantity of controlled hole injection with time can be suppressed.

Further, in a light-emitting element of the present invention, the layer215 for controlling the electron transport is also provided. In thismanner, electrons injected from the second electrode 204 pass throughthe electron-injecting layer 217 and the electron-transporting layer 216to be injected into the layer 215 for controlling the electrontransport. Here, the layer 215 for controlling the electron transporthas a structure in which the fourth organic compound having ahole-transporting property is added to the third organic compound havingan electron-transporting property. Therefore, the rate of the electronsinjected into the layer 215 for controlling the electron transport isretarded, and electron injection into the light-emitting layer 214 iscontrolled. As a result, the possibility that electrons may reach anddeteriorate the electron-transporting layer 212 is lowered. Similarly,as for holes, the possibility that holes may reach and deteriorate theelectron-transporting layer 216 is further lowered since the layer 215for controlling the electron transport includes the third organiccompound having an electron-transporting property. Note that it isimportant in the present invention that an organic compound whichreduces an electron-transporting property is added to an organiccompound having an electron-transporting property, instead of justapplying a substance with low electron mobility in the layer 215 forcontrolling the electron transport. When such a structure is employed,in addition to just controlling electron injection into thelight-emitting layer 214, change in the quantity of the controlled holeinjection with time can be suppressed.

Therefore, by controlling the quantity of injection of both carriers ofholes and electrons into the light-emitting layer, a light-emittingelement of the present invention can prevent a phenomenon that carrierbalance is lost and recombination probability is reduced with time.Thus, the lifetime of the element can be improved (luminance decay withtime can be suppressed).

Further, as an effect of the layer 213 for controlling the holetransport, improvement in light emission efficiency can be given. In acase of a conventional element where the layer 213 for controlling thehole transport is not provided, most of the holes injected from thefirst electrode 202 are injected into the light-emitting layer 214without the transport being controlled. In a case where thelight-emitting layer 214 is a layer having an electron-transportingproperty, that is, in a case where the material which has the highestweight percent in the light-emitting layer 214 has anelectron-transporting property, a light-emitting region is formed in thevicinity of the interface between the light-emitting layer 214 and thehole-transporting layer 212. In addition, there is a possibility thatcations are generated by excessive holes in the vicinity of theinterface between the light-emitting layer 214 and the hole-transportinglayer 212. Since a cation serves as a quencher, light emissionefficiency decreases due to cations generated in the vicinity of thelight-emitting region.

However, by providing the layer 213 for controlling the hole transportshown in this embodiment mode, formation of cations generated byexcessive holes in the light-emitting layer 214 and in the vicinity ofthe light-emitting layer 214 can be suppressed, and decrease in lightemission efficiency can be suppressed. Therefore, a light-emittingelement with high light emission efficiency can be obtained.

As described above, the light-emitting element shown in this embodimentmode includes a layer for controlling the carrier transport. Since thelayer for controlling the carrier transport includes two or more kindsof substances, carrier balance can be controlled precisely by control ofcombination, the mixture ratio, the film thickness, or the like of thesubstances.

Further, since the carrier balance can be controlled by controllingcombination, the mixture ratio, the film thickness, or the like of thesubstances, control of the carrier balance can be easier than aconventional light-emitting element. That is, even if a physicalproperty of the substance itself is not changed, the carrier transportcan be controlled by controlling the mixture ratio, the film thickness,or the like.

Among two or more kinds of substances included in the layer forcontrolling the carrier transport, an organic compound which has lowerweight percent than other substances is used for controlling the carriertransport. That is, the carrier transport can be controlled by acomponent which has lower weight percent than other components includedin the layer for controlling the carrier transport. Thus, alight-emitting element with a long lifetime, which does not easilydeteriorate, can be realized. That is, change in carrier balance ishardly caused in the light-emitting element as compared with a casewhere carrier balance is controlled by a single substance. For example,when the carrier transport is controlled by a layer formed of a singlesubstance, a balance of the whole layer is changed by a partial changein morphology, partial crystallization, or the like; therefore, thelayer easily deteriorates with time. However, as shown in thisembodiment mode, the carrier transport is controlled by a componentwhich has lower weigh percent than other components included in thelayer for controlling the carrier transport, whereby an effect of changein morphology, crystallization, aggregation, or the like is reduced, andthus deterioration with time is hardly caused. Thus, a light-emittingelement with a long lifetime can be obtained in which reduction ofcarrier balance with time and reduction in light emission efficiencywith time is hardly caused.

Moreover, by controlling the carrier transport at opposing sides of thelight-emitting layer, effect of change in morphology, crystallization,aggregation, or the like is further reduced, and thus deterioration withtime is hardly caused. Thus, a light-emitting element with a longlifetime can be obtained in which reduction in light emission efficiencywith time is hardly caused.

In addition, by controlling the carrier transport at opposing sides ofthe light-emitting layer, a light-emitting element with a long lifetimecan be obtained without depending on a carrier-transporting property ofthe light-emitting layer. Therefore, a material for the light-emittinglayer can be chosen from a wider range, and the light-emitting elementcan be designed more flexibly.

Note that this embodiment mode can be appropriately combined withanother embodiment mode.

Embodiment Mode 2

In this embodiment mode, a mode of a light-emitting element in which aplurality of light-emitting units according to the present invention arestacked (hereinafter this light-emitting element is referred to as astacked-type light-emitting element) will be described with reference toFIG. 6. This light-emitting element is a stacked-type light-emittingelement including a plurality of light-emitting units between a firstelectrode and a second electrode. Each structure of the light-emittingunits can be similar to the structure described in Embodiment Mode 1. Inother words, the light-emitting element described in Embodiment Mode 1is a light-emitting element having one light-emitting unit. In thisembodiment mode, a light-emitting element having a plurality oflight-emitting units will be described.

In FIG. 6, a first light-emitting unit 511 and a second light-emittingunit 512 are stacked between a first electrode 501 and a secondelectrode 502. As the first electrode 501 and the second electrode 502,similar electrodes to the electrode shown in Embodiment Mode 1 can beemployed. Note that the first light-emitting unit 511 and the secondlight-emitting unit 512 may have the same structure or differentstructures, and as the structures, a similar structure to the structureshown in Embodiment Mode 1 can be employed.

A charge generation layer 513 includes a composite material in which asubstance having an acceptor property is mixed into an organic compound.This composite material of an organic compound and a substance having anacceptor property is the composite material described in Embodiment Mode1 and includes 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane(abbreviation: F₄-TCNQ), or metal oxide such as vanadium oxide,molybdenum oxide, or tungsten oxide as a substance having an acceptorproperty. As an organic compound, any of various compounds such as anaromatic amine compound, a carbazole derivative, an aromatichydrocarbon, a high molecular compound, an oligomer, a dendrimer, and apolymer can be used. Note that the organic compound having a holemobility of 10⁻⁶ cm²/Vs or higher is preferably employed as an organiccompound. However, other substances than the materials described abovemay also be used as long as the hole-transporting properties thereof arehigher than the electron-transporting properties thereof. A composite ofan organic compound and metal oxide is superior in a carrier-injectingproperty and a carrier-transporting property, and accordingly,low-voltage driving and low-current driving can be realized.

Note that the charge generation layer 513 may be formed with acombination of the composite material of an organic compound and asubstance having an acceptor property, and other material. For example,the charge generation layer 513 may be formed with a combination of alayer including the composite material of an organic compound and metaloxide, and a layer including one compound selected from substanceshaving an electron-donating property and a compound having a highelectron-transporting property. Further, the charge generation layer 513may be formed with a combination of a layer including the compositematerial of an organic compound and metal oxide, and a transparentconductive film. Furthermore, electrode materials shown in EmbodimentMode 1 can be used for the charge generation layer. Note that a layerhaving a high light-transmitting property is preferably used as thecharge generation layer in terms of light extraction efficiency.

In any case, the charge generation layer 513 interposed between thefirst light-emitting unit 511 and the second light-emitting unit 512 isacceptable as long as electrons are injected into a light-emitting uniton one side and holes are injected into a light-emitting unit on theother side when voltage is applied to the first electrode 501 and thesecond electrode 502. For example, in a case of applying voltage so thatpotential of the first electrode is higher than potential of the secondelectrode, any structure is acceptable for the charge generation layer513 as long as the charge generation layer 513 injects electrons andholes into the first light-emitting unit 511 and the secondlight-emitting unit 512, respectively.

Although the light-emitting element having two light-emitting units isdescribed in this embodiment mode, a light-emitting element in whichthree or more light-emitting units are stacked can be employed in asimilar way. Like the light-emitting element of this embodiment mode, aplurality of light emitting units are disposed between a pair ofelectrodes so as to be partitioned with the charge generation layer, andaccordingly, the element with a long lifetime in a high luminance regioncan be realized while keeping low current density. In a case where thelight-emitting element is applied to lighting as an application example,voltage drop due to resistance of an electrode material can be reduced.Accordingly, light can be uniformly emitted in a large area. Moreover, alight-emitting device of low power consumption, which can be driven atlow voltage, can be achieved.

The light-emitting units emit light of different colors from each other,thereby obtaining light emission of a desired color as the wholelight-emitting element. For example, in a light-emitting element havingtwo light-emitting units, by making the emission colors of the firstlight-emitting unit and the second light-emitting unit complementarycolors, the light-emitting element which emits white light as the wholeelement can be obtained. Note that “complementary colors” refer tocolors which can produce an achromatic color when mixed. That is, whitelight emission can be obtained by mixing light obtained from substancesemitting light of complementary colors. The same can be applied to alight-emitting element having three light-emitting units. For example,when the first light-emitting unit emits red light, the secondlight-emitting unit emits green light, and the third light-emitting unitemits blue light, white light can be emitted as the whole light-emittingelement.

Note that this embodiment mode can be appropriately combined withanother embodiment mode.

Embodiment Mode 3

In this embodiment mode, a light-emitting device having a light-emittingelement of the present invention will be described.

A light-emitting device having a light-emitting element of the presentinvention in a pixel portion is described in this embodiment mode withreference to FIGS. 7A and 7B. Note that FIG. 7A is a top viewillustrating the light-emitting device and FIG. 7B is a cross-sectionalview of FIG. 7A taken along lines A-A′ and B-B′. This light-emittingdevice includes a driver circuit portion (source side driver circuit)601, a pixel portion 602, and a driver circuit portion (gate side drivercircuit) 603, which are indicated by dotted lines, in order to controlthe light emission of the light-emitting element. Further, referencenumeral 604 indicates a sealing substrate and reference numeral 605indicates a sealing material. A space 607 is provided inside of aportion surrounded by the sealing material 605.

Note that a leading wiring 608 is a wiring for transmitting signalsinput in the source side driver circuit 601 and the gate side drivercircuit 603. The leading wiring 608 receives video signals, clocksignals, start signals, reset signals, and the like from an FPC(flexible printed circuit) 609 that serves as an external inputterminal. Although only an FPC is illustrated here, this FPC may beprovided with a printed wiring board (PWB). The light-emitting device inthis specification includes not only a light-emitting device itself butalso a light-emitting device with an FPC or a PWB attached thereto.

Then, a cross-sectional structure is described with reference to FIG.7B. The driver circuit portions and the pixel portion are provided overan element substrate 610. In FIG. 7B, only the source side drivercircuit 601, which is the driver circuit portion, and one pixel of thepixel portion 602 are illustrated.

Note that a CMOS circuit which is a combination of an N-channel TET 623and a P-channel TFT 624 is provided in the source side driver circuit601. The driver circuit may be formed by various CMOS circuits, PMOScircuits, or NMOS circuits. In this embodiment mode, a driver-integratedtype in which a driver circuit is formed over a substrate is described;however, the present invention is not limited to this, and the drivercircuit can be formed outside the substrate.

The pixel portion 602 includes a plurality of pixels each having aswitching TFT 611, a current controlling TFT 612, and a first electrode613 that is electrically connected to a drain of the current controllingTFT 612. Note that an insulator 614 is formed to cover the edge of thefirst electrode 613. Here, a positive photosensitive acrylic resin filmis used to form the insulator 614.

Further, in order to improve the coverage, the insulator 614 is providedsuch that either an upper edge portion or a lower edge portion of theinsulator has a curved surface with a curvature. For example, whenpositive photosensitive acrylic is used as a material for the insulator614, it is preferable that only an upper edge portion of the insulator614 have a curved surface with a radius of curvature (0.2 μm to 3 μm).The insulator 614 can be formed using either negative type that becomesinsoluble in an etchant by light irradiation, or positive type thatbecomes dissoluble in an etchant by light irradiation.

An EL layer 616 and a second electrode 617 are formed over the firstelectrode 613. Here, various metals, alloys, electrically conductivecompounds, or mixtures thereof can be used for a material of the firstelectrode 613. If the first electrode is used as an anode, it ispreferable that the first electrode be formed using a metal, an alloy,an electrically conductive compound, a mixture thereof with a high workfunction (a work function of 4.0 eV or higher) among such materials. Forexample, the first electrode 613 can be formed using a single-layer filmsuch as an indium tin oxide film containing silicon, an indium zincoxide film, a titanium nitride film, a chromium film, a tungsten film, aZn film, a Pt film, or the like; a stacked film of a titanium nitridefilm and a film containing aluminum as its main component; or athree-layer structure of a titanium nitride film, a film containingaluminum as its main component, and a titanium nitride film. Note thatwhen a stacked structure is employed, the first electrode 613 has lowresistance as a wiring, forms a favorable ohmic contact, and can serveas an anode.

The EL layer 616 is formed by various methods such as an evaporationmethod using an evaporation mask, an inkjet method, a spin coatingmethod, or the like. The EL layer 616 includes the layer for controllingthe carrier transport shown in Embodiment Mode 1 or Embodiment Mode 2.Any of a low molecular compound, a high molecular compound, an oligomer,or a dendrimer may be employed as a material for the EL layer 616. Asthe material for the EL layer, not only an organic compound but also aninorganic compound may be used.

As the material for the second electrode 617, various types of metals,alloys, electrically conductive compounds, mixtures thereof, or the likecan be used. If the second electrode is used as a cathode, it ispreferable that the second electrode be formed using an alloy, anelectrically conductive compound, a mixture thereof with a low workfunction (a work function of 3.8 eV or lower) among such materials. Forexample, elements belonging to Group 1 and Group 2 of the periodictable, that is, alkali metals such as lithium (Li) and cesium (Cs) andalkaline earth metals such as magnesium (Mg), calcium (Ca), andstrontium (Sr); alloys thereof (MgAg, AlLi); and the like can be given.In a case where light generated in the EL layer 616 is transmittedthrough the second electrode 617, the second electrode 617 may also beformed by using a stacked layer of a thin metal film with a reduced filmthickness and a transparent conductive film (indium tin oxide (ITO),indium tin oxide containing silicon or silicon oxide, indium zinc oxide(IZO), indium oxide containing tungsten oxide and zinc oxide (IWZO), orthe like).

By attaching the sealing substrate 604 to the element substrate 610 withthe sealing material 605, the light-emitting element 618 is provided inthe space 607 which is surrounded by the element substrate 610, thesealing substrate 604, and the sealing material 605. The space 607 maybe filled with a filler, and may be filled with an inert gas (such asnitrogen and argon), the sealing material 605, or the like.

As the sealing material 605, an epoxy-based resin is preferably used. Inaddition, it is desirable to use a material that allows permeation ofmoisture or oxygen as little as possible. As the sealing substrate 604,a plastic substrate formed of FRP (fiberglass-reinforced plastics), PVF(polyvinyl fluoride), polyester, acrylic, or the like can be usedbesides a glass substrate or a quartz substrate.

As described above, the light-emitting device having a light-emittingelement of the present invention can be obtained.

A light-emitting device of the present invention has the light-emittingelement shown in Embodiment Mode 1 or Embodiment Mode 2. Thus, alight-emitting device with high light emission efficiency can beobtained.

In addition, since the light-emitting element with high light emissionefficiency is included, a light-emitting device with low powerconsumption can be obtained.

Furthermore, since a light-emitting element with less deterioration anda long lifetime is included, a light-emitting device with a longlifetime can be obtained.

As described above, an active matrix light-emitting device that controlsdriving of a light-emitting element with a transistor is described inthis embodiment mode; however, a passive matrix light-emitting devicemay be used. FIGS. 8A and 8B illustrate a perspective view of a passivematrix light-emitting device manufactured according to the presentinvention. Note that FIG. 8A is a perspective view of the light-emittingdevice and FIG. 8B is a cross-sectional view of FIG. 8A taken along aline X-Y. In FIGS. 8A and 8B, an EL layer 955 is provided between anelectrode 952 and an electrode 956 over a substrate 951. The edge of theelectrode 952 is covered with an insulating layer 953. A partition layer954 is provided over the insulating layer 953. The sidewalls of thepartition layer 954 slope so that the distance between one sidewall andthe other sidewall is gradually reduced toward the surface of thesubstrate. In other words, a cross section taken in the direction of theshort side of the partition layer 954 is quadrilateral, and the lowerside (a side in contact with the insulating layer 953) is shorter thanthe upper side (an opposite side of the lower side). A cathode can bepatterned by providing the partition layer 954 in this manner. Inaddition, in a passive matrix light-emitting device, a light-emittingdevice with high light emission efficiency can be obtained by includinga light-emitting element with high light emission efficiency of thepresent invention.

A light-emitting device of the present invention has the light-emittingelement shown in Embodiment Mode 1 or Embodiment Mode 2. Thus, alight-emitting device with high light emission efficiency can beobtained.

In addition, since the light-emitting element with high light emissionefficiency is included, a light-emitting device with low powerconsumption can be obtained.

Furthermore, since a light-emitting element with less deterioration anda long lifetime is included, a light-emitting device with a longlifetime can be obtained.

Note that this embodiment mode can be appropriately combined withanother embodiment mode.

Embodiment Mode 4

In this embodiment mode, an electronic device of the present inventionwhich includes the light-emitting device shown in Embodiment Mode 3 willbe described. An electronic device of the present invention has thelight-emitting element described in Embodiment Mode 1 or Embodiment Mode2, and a display portion with high light emission efficiency and lowpower consumption. In addition, the display portion has a long lifetime.

As an electronic device manufactured using a light-emitting device ofthe present invention, cameras such as a video camera and a digitalcamera, a goggle type display, a navigation system, an audio reproducingdevice (a car audio set, an audio component set, or the like), acomputer, a game machine, a portable information terminal (a mobilecomputer, a cellular phone, a portable game machine, an electronic bookreader, or the like), an image reproducing device provided with arecording medium (specifically, a device provided with a display devicethat can reproduce a recording medium and display the image such as adigital versatile disc (DVD)), and the like are given. Specific examplesof these electronic devices are illustrated in FIGS. 9A to 9D.

FIG. 9A illustrates a television device of this embodiment mode thatincludes a housing 9101, a support 9102, a display portion 9103, speakerportions 9104, a video input terminal 9105, and the like. In the displayportion 9103 of this television device, light-emitting elements similarto those described in Embodiment Modes 1 or Embodiment Mode 2 arearranged in matrix. One feature of the light-emitting element is thatlight emission efficiency is high and power consumption is low. Further,the light-emitting element has a long lifetime. The display portion 9103which includes the light-emitting element has similar features.Therefore, in this television device, image quality is hardlydeteriorated and low power consumption is achieved. With such features,deterioration compensation function and a power supply circuit can besignificantly reduced or downsized in the television device; therefore,reduction in size and weight of the housing 9101 and the support 9102can be achieved. In the television device of this embodiment mode, lowpower consumption, high image quality, and reduction in size and weightare achieved; therefore, a product which is suitable for livingenvironment can be provided.

FIG. 9B illustrates a computer of this embodiment mode that includes amain body 9201, a housing 9202, a display portion 9203, a keyboard 9204,an external connection port 9205, a pointing device 9206, and the like.In the display portion 9203 of this computer, light-emitting elementssimilar to those described in Embodiment Mode 1 or Embodiment Mode 2 arearranged in matrix. One feature of the light-emitting element is thatlight emission efficiency is high and power consumption is low. Further,the light-emitting element has a long lifetime. The display portion 9203which includes the light-emitting elements has similar features.Therefore, in the computer, image quality is hardly deteriorated andlower power consumption is achieved. With such features, deteriorationcompensation function and a power supply circuit can be significantlyreduced or downsized in the computer; therefore, reduction in size andweight of the main body 9201 and the housing 9202 can be achieved. Inthe computer of this embodiment mode, low power consumption, high imagequality, and reduction in size and weight are achieved; therefore, aproduct which is suitable for environment can be provided. Moreover, thecomputer can be carried and the computer having the display portionwhich has strong resistance to external impact when being carried can beprovided.

FIG. 9C illustrates a cellular phone of this embodiment mode thatincludes a main body 9401, a housing 9402, a display portion 9403, anaudio input portion 9404, an audio output portion 9405, operation keys9406, an external connection port 9407, an antenna 9408, and the like.In the display portion 9403 of this cellular phone, light-emittingelements similar to those described in Embodiment Mode 1 or EmbodimentMode 2 are arranged in matrix. One feature of the light-emitting elementis that light emission efficiency is high and power consumption is low.Further, the light-emitting element has a long lifetime. The displayportion 9403 which includes the light-emitting elements has similarfeatures. Therefore, in the cellular phone, image quality is hardlydeteriorated and lower power consumption is achieved. With suchfeatures, deterioration compensation function and a power supply circuitcan be significantly reduced or downsized in the cellular phone;therefore, reduction in size and weight of the main body 9401 and thehousing 9402 can be achieved. In the cellular phone of this embodimentmode, low power consumption, high image quality, and reduction in sizeand weight are achieved; therefore, a product which is suitable forbeing carried can be provided. Further, the present invention canprovide a product, a display portion of which is resistant to impacteven when being carried.

FIG. 9D illustrates a camera that includes a main body 9501, a displayportion 9502, a housing 9503, an external connection port 9504, a remotecontrol receiving portion 9505, an image receiving portion 9506, abattery 9507, an audio input portion 9508, operation keys 9509, aneyepiece portion 9510, and the like. In the display portion 9502 of thiscamera, light-emitting elements similar to those described in EmbodimentModes 1 or Embodiment Mode 2 are arranged in matrix. One feature of thelight-emitting element is that light emission efficiency is high andpower consumption is low. Further, the light-emitting element has a longlifetime. The display portion 9502 which includes the light-emittingelements has similar features. Therefore, in the camera, image qualityis hardly deteriorated and lower power consumption is achieved. Withsuch features, deterioration compensation function and a power supplycircuit can be significantly reduced or downsized in the camera, therebyachieving reduction in size and weight of the main body 9501. In thecamera of this embodiment mode, low power consumption, high imagequality, and reduction in size and weight are achieved; therefore, aproduct which is suitable for being carried can be provided. Further,the present invention can provide a product, a display portion of whichis resistant to impact even when being carried.

FIG. 10 illustrates an audio reproducing device, specifically, a caraudio system. The audio reproducing device includes a main body 701, adisplay portion 702, and operation switches 703 and 704. The displayportion 702 can be realized by the light-emitting device (passive matrixor active matrix) described in Embodiment Mode 2. Further, the displayportion 702 may be formed using a segment type light-emitting device. Inany case, the use of a light-emitting element of the present inventionmakes it possible to form a bright display portion with a long lifetimewhile achieving low power consumption which uses a vehicle power source(12 to 42 V). Further, although this embodiment mode describes an in-caraudio system, a light-emitting device of the present invention may alsobe used in portable audio systems or audio systems for home use.

FIG. 11 illustrates a digital player as one example of that. The digitalplayer illustrated in FIG. 11 includes a main body 710, a displayportion 711, a memory portion 712, an operation portion 713, earphones714, and the like. Note that a pair of headphones or a pair of wirelessearphones can be used instead of the pair of earphones 714. The displayportion 711 can be realized by the light-emitting device (passive matrixor active matrix) described in Embodiment Mode 2. Further, the displayportion 711 may be formed using a segment type light-emitting device. Inany case, by using a light-emitting element of the present invention, adisplay portion can be formed that is capable of displaying images evenwhen using a secondary battery (a nickel-hydrogen battery or the like),has a long lifetime, is bright, and achieves low power consumption. Asthe memory portion 712, a hard disk or a nonvolatile memory is used. Forexample, a NAND type nonvolatile memory with a recording capacity of 20to 200 gigabytes (GB) is used, and by operating the operation portion713, an image or a sound (music) can be recorded and reproduced. Notethat in the display portion 711, white characters are displayed againsta black background, and thus, power consumption can be reduced. This isparticularly effective for portable audio systems.

As described above, the applicable range of the light-emitting devicemanufactured by applying the present invention is so wide that thelight-emitting device is applicable to electronic devices in variousfields. By applying the present invention, an electronic device whichhas a display portion consuming low power and having high reliabilitycan be manufactured.

A light-emitting device to which the present invention is applied has alight-emitting element with high light emission efficiency, and can alsobe used as a lighting device. One mode of using a light-emitting elementto which the present invention is applied as a lighting device isdescribed with reference to FIG. 12.

FIG. 12 illustrates an example of a liquid crystal display device usinga light-emitting device of the present invention as a backlight. Theliquid crystal display device illustrated in FIG. 12 includes a housing901, a liquid crystal layer 902, a backlight 903, and a housing 904. Theliquid crystal layer 902 is connected to a driver IC 905. Alight-emitting device of the present invention is used as the backlight903, and current is supplied through a terminal 906.

By using a light-emitting device of the present invention as a backlightof the liquid crystal display device, the backlight can have high lightemission efficiency. In addition, a backlight with a long lifetime canbe obtained. A light-emitting device of the present invention is a planeemission type lighting device, and can have a large area. Therefore, thebacklight can have a large area, and a liquid crystal display devicehaving a large area can be obtained. Furthermore, a light-emittingdevice of the present invention has a thin shape and consumes low power;therefore, a thin shape and low power consumption of a display devicecan also be achieved.

FIG. 13 illustrates an example in which a light-emitting device to whichthe present invention is applied is used as a desk lamp, which is one oflighting devices. The desk lamp illustrates in FIG. 13 includes ahousing 2001 and a light source 2002, and a light-emitting device of thepresent invention is used as the light source 2002. Since alight-emitting device of the present invention has a long lifetime, thedesk lamp can also have a long lifetime.

FIG. 14 illustrates an example in which a light-emitting device to whichthe present invention is applied is used as an interior lighting device3001. Since a light-emitting device of the present invention can alsohave a large area, a light-emitting device of the present invention canbe used as a lighting device having a large emission area. Moreover,since a light-emitting device of the present invention has a longlifetime, the lighting device can also have a long lifetime. Atelevision device 3002 of the present invention such as that illustratedin FIG. 9A may be placed in a room where a light-emitting device towhich the invention is applied is used as the interior lighting device3001, and public broadcasting or movies can be watched there. In such acase, since both of the devices have long lifetimes, frequency ofreplacement of the lighting device and the television device can bereduced, and environmental load can be reduced.

Note that this embodiment can be appropriately combined with anotherembodiment mode.

This application is based on Japanese Patent Application Serial No.2007-243273 filed with Japan Patent Office on Sep. 20, 2007, the entirecontents of which are hereby incorporated by reference.

1. A light-emitting element comprising: a light-emitting layer betweenan anode and a cathode; a first layer between the light-emitting layerand the anode; and a second layer between the light-emitting layer andthe cathode, wherein the first layer comprises a first organic compoundhaving a hole-transporting property and a second organic compound havinga hole-trapping property, wherein a weight percent of the first organiccompound is higher than a weight percent of the second organic compound,wherein the second layer comprises a third organic compound having anelectron-transporting property and a fourth organic compound having ahole-transporting property, and wherein a weight percent of the thirdorganic compound is higher than a weight percent of the fourth organiccompound.
 2. The light-emitting element according to claim 1, wherein ahighest occupied molecular orbital level of the second organic compoundis higher than a highest occupied molecular orbital level of the firstorganic compound by 0.3 eV or more.
 3. The light-emitting elementaccording to claim 1, wherein the first organic compound is an aromaticamine compound.
 4. The light-emitting element according to claim 1,wherein a thickness of the first layer is 5 nm to 20 nm, inclusive. 5.The light-emitting element according to claim 1, wherein the first layeris in contact with the light-emitting layer.
 6. The light-emittingelement according to claim 1, wherein a difference between a lowestunoccupied molecular orbital level of the third organic compound and alowest unoccupied molecular orbital level of the fourth organic compoundis less than 0.3 eV.
 7. The light-emitting element according to claim 1,wherein the third organic compound is a metal complex, and wherein thefourth organic compound is an aromatic amine compound.
 8. Thelight-emitting element according to claim 1, wherein P₁/P₂≧3 orP₁/P₂≦0.33 is satisfied where P₁ is a dipole moment of the third organiccompound and P₂ is a dipole moment of the fourth organic compound. 9.The light-emitting element according to claim 1, wherein a thickness ofthe second layer is from 5 nm to 20 nm, inclusive.
 10. Thelight-emitting element according to claim 1, wherein the second layer isin contact with the light-emitting layer.
 11. An electronic devicecomprising the light-emitting element according to claim 1, theelectronic device further comprising a display portion, wherein thedisplay portion is provided with the light-emitting element.
 12. Alight-emitting element comprising: a first electrode; a secondelectrode; and a plurality of light-emitting units between the firstelectrode and the second electrode, wherein at least one of theplurality of light-emitting units comprises: a first layer comprising afirst organic compound having a hole-transporting property and a secondorganic compound having a hole-trapping property; a second layercomprising a third organic compound having an electron-transportingproperty and a fourth organic compound having a hole-transportingproperty; and a light-emitting layer interposed between the first layerand the second layer, wherein a weight percent of the first organiccompound is higher than a weight percent of the second organic compound,and wherein a weight percent of the third organic compound is higherthan a weight percent of the fourth organic compound, and wherein theplurality of light-emitting units are stacked.
 13. A light-emittingelement according to claim 12, wherein each of the light-emitting unitsis the same structure.
 14. A light-emitting element according to claim12, wherein the light-emitting element emits white light.
 15. Thelight-emitting element according to claim 12, wherein a highest occupiedmolecular orbital level of the second organic compound is higher than ahighest occupied molecular orbital level of the first organic compoundby 0.3 eV or more.
 16. The light-emitting element according to claim 12,wherein the first organic compound is an aromatic amine compound. 17.The light-emitting element according to claim 12, wherein the firstlayer is in contact with the light-emitting layer.
 18. Thelight-emitting element according to claim 12, wherein a differencebetween a lowest unoccupied molecular orbital level of the third organiccompound and a lowest unoccupied molecular orbital level of the fourthorganic compound is less than 0.3 eV.
 19. The light-emitting elementaccording to claim 12, wherein the third organic compound is a metalcomplex, and wherein the fourth organic compound is an aromatic aminecompound.
 20. The light-emitting element according to claim 12, whereinP₁/P₂≧3 or P₁/P₂≦0.33 is satisfied where P₁ is a dipole moment of thethird organic compound and P₂ is a dipole moment of the fourth organiccompound.
 21. The light-emitting element according to claim 12, whereinthe second layer is in contact with the light-emitting layer.
 22. Alight-emitting element comprising: an anode; a hole-injecting layer overthe anode; a light-emitting layer over the hole-injecting layer; anelectron-injecting layer over the light-emitting layer; and a cathodeover the electron-injecting layer, wherein a hole-transporting layer anda first layer are provided between the hole-injecting layer and thelight-emitting layer, wherein an electron-transporting layer and asecond layer are provided between the electron-injecting layer and thelight-emitting layer, wherein the first layer comprises a first organiccompound having a hole-transporting property and a second organiccompound having a hole-trapping property, wherein a weight percent ofthe first organic compound is higher than a weight percent of the secondorganic compound, wherein the second layer comprises a third organiccompound having an electron-transporting property and a fourth organiccompound having a hole-transporting property, and wherein a weightpercent of the third organic compound is higher than a weight percent ofthe fourth organic compound.
 23. A light-emitting element according toclaim 22, wherein the first layer is over the hole-transporting layer,and wherein the electron-transporting layer is over the second layer.24. A light-emitting element according to claim 22, wherein thehole-transporting layer is over the first layer, and wherein theelectron-transporting layer is over the second layer.
 25. Alight-emitting element according to claim 22, wherein the first layer isover the hole-transporting layer, and wherein the second layer is overthe electron-transporting layer.
 26. A light-emitting element accordingto claim 22, wherein the hole-transporting layer is over the firstlayer, and wherein the second layer is over the electron-transportinglayer.
 27. The light-emitting element according to claim 22, wherein ahighest occupied molecular orbital level of the second organic compoundis higher than a highest occupied molecular orbital level of the firstorganic compound by 0.3 eV or more.
 28. The light-emitting elementaccording to claim 22, wherein the first organic compound is an aromaticamine compound.
 29. The light-emitting element according to claim 22,wherein a thickness of the first layer is 5 nm to 20 nm, inclusive. 30.The light-emitting element according to claim 22, wherein the firstlayer is in contact with the light-emitting layer.
 31. Thelight-emitting element according to claim 22, wherein a differencebetween a lowest unoccupied molecular orbital level of the third organiccompound and a lowest unoccupied molecular orbital level of the fourthorganic compound is less than 0.3 eV.
 32. The light-emitting elementaccording to claim 22, wherein the third organic compound is a metalcomplex, and wherein the fourth organic compound is an aromatic aminecompound.
 33. The light-emitting element according to claim 22, whereinP₁/P₂≦3 or P₁/P₂≦0.33 is satisfied where P₁ is a dipole moment of thethird organic compound and P₂ is a dipole moment of the fourth organiccompound.
 34. The light-emitting element according to claim 22, whereina thickness of the second layer is from 5 nm to 20 nm, inclusive. 35.The light-emitting element according to claim 22, wherein the secondlayer is in contact with the light-emitting layer.
 36. An electronicdevice comprising the light-emitting element according to claim 22, theelectronic device further comprising a display portion, wherein thedisplay portion is provided with the light-emitting element.
 37. Thelight-emitting element according to claim 1, wherein the first organiccompound and the second organic compound are mixed, and wherein thethird organic compound and the fourth organic compound are mixed. 38.The light-emitting element according to claim 12, wherein the firstorganic compound and the second organic compound are mixed, and whereinthe third organic compound and the fourth organic compound are mixed.39. The light-emitting element according to claim 22, wherein the firstorganic compound and the second organic compound are mixed, and whereinthe third organic compound and the fourth organic compound are mixed.40. A lighting device comprising the light-emitting element according toclaim
 1. 41. A lighting device comprising the light-emitting elementaccording to claim
 12. 42. A lighting device comprising thelight-emitting element according to claim 22.